Amazon offers Prime members the Gro Company Gro-Anywhere Blackout Blind for $31.24 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $7.27.) That's the lowest total price we could find by $19, outside of the mention below. It attaches with suction cups and measures 51" x 74".
Don't mind picking it up? Walmart charges the same with free in-store pickup.